The present invention relates to a method and apparatus of cooling exhaust gas from a refuse and/or waste incinerator (hereinafter referred to as "incinerator exhaust gas"). More particularly, the present invention relates to a method and apparatus of cooling incinerator exhaust gas, which particularly suppress regeneration of dioxins and can remove harmful substances, such as dioxins or mercury, together with hydrogen chloride gas, sulfur oxide gas, dust, fly ash, and the like, in high efficiency with a bag filter.
Exhaust gases discharged from municipal refuse incinerator contain minor components, such as smoke dust, hydrogen chloride (HCl), SOx, NOx, heavy metals including mercury, or dioxin. From the standpoint of environmental protection, it is necessary to remove those harmful substances.
Of those, dioxins (general name of PCDD: polychloro-dibenzodioxin, and PCDF: polychlorodibenzofuran) have extremely strong toxicity, and it is also reported that dioxins have carcinogenesis. Therefore, collection and removal of dioxins is extremely important.
A conventional example of a municipal refuse incinerator plant having an exhaust gas treating apparatus is shown in FIG. 11.
In the apparatus shown in FIG. 11, refuse gathered in a dump pit 1 is sent to an incinerator 2 by, for example, a crane (not shown in the drawings). Refuse is incinerated in the incinerator 2, and then completely combusted in a secondary combustion chamber 3 by secondary air. Ash after combustion in the incinerator 2 is discharged outside from an ash outlet 11. Exhaust gas, generated as a result of complete combustion of refuse in the secondary combustion chamber 3, is subjected to heat recovery by waste heat boiler 4 and waste heat reclaimer (pre-heater) 5, and then reaches a quenching reaction tower 6.
In the quenching reaction tower 6, slaked lime slurry is sprayed from slaked lime tank 7, so that hydrogen chloride (HCl) and sulfur oxide (SOx) contained in the exhaust gas are removed. The exhaust gas is then led to bag filter 8 at downstream side, where smoke dust and/or fly ash (hereinafter referred to as "dust"), HCl, SOx, heavy metals and dioxins, which remain in the exhaust gas, are removed. Reference numeral 9 is an induced draft fan which suctions the exhaust gas after treatment as mentioned above, and discharges the same into the atmosphere through a chimney 10.
However, even where the exhaust gas from the incinerator is treated with the above-mentioned exhaust gas treating apparatus, there is the possibility that dioxins cannot be reduced to the desired low concentration. Dioxins generated in the course of incineration of refuse are almost decomposed in the secondary combustion chamber, but it is necessary to decrease the temperature of the incinerator exhaust gas from a high temperature of about 350 to 900.degree. C. to a low temperature in each step of heat recovery step, cooling reaction step, and dust collection step, which are exhaust gas treatment steps. However, regeneration of dioxins having strong toxicity tends to occur particularly in the vicinity of 300.degree. C. Therefore, in the above-mentioned conventional exhaust gas treatment apparatus, dioxins regenerate in each step, and as a result, such a problem arises in that dioxins cannot be collected and removed to the desired low concentration.
Further, it is difficult in the conventional cooling tower to decrease a temperature to 150.degree. C. or less, and the temperature is generally about 150 to 200.degree. C. As a result, the problem occurs in that dioxins cannot be collected and removed to the desired low concentration.
The waste heat reclaimer, as a cooling technique of exhaust gas, has the problem that cooling time is somewhat long, so that flow of gas becomes partially heterogeneous, and also dust is deposited and accumulated on the surface of instruments. Further, if there is a change in temperature or gas flow rate, an outlet temperature changes. As a result, generation of dioxins increases, and water injection cooling means are required in order to control the temperature.
If the water injection cooling tower is used as exhaust gas cooling means, cooling is conducted with rough water droplets having an average particle size of 150 .mu.m or more, generally 250 .mu.m or more, so that the temperature of gas cannot be controlled, and dust is converted to a wet or a slurry state. As a result, if the outlet temperature is as low as 150.degree. C. or less, it is impossible to use a bag filter, since it becomes difficult to treat the dust. Further, there is a method of spraying water or slurry to duct or tower portion as water spraying cooling means, but such a method involves the defect that gas cannot uniformly be cooled.
Furthermore, in order to remove harmful substances contained in exhaust gas, particularly dioxin or mercury, a powder for removing harmful substances (hereinafter referred to as "harmful substance removing powder") is introduced into an exhaust pipe at a post-stream side of the water injection cooling tower to mix with exhaust gas in the exhaust pipe, and the harmful substances are then collected and removed with dust collector, for example, using bag filter or electrostatic precipitator. However, contact and retention time with the exhaust gas can be as short as 10 seconds or less, and removal efficiency is insufficient.
Regarding the above-described exhaustion suppression of dioxins, the Ministry of Health and Welfare in Japan decided `Guideline on Prevention of Generation of Dioxins` in 1990. It was therefore expected that exhaust concentration of dioxins was about 0.5 ng-TEQ (Toxicity Equivalent Conversion Concentration)/Nm.sup.3 or less. The Ministry of Health and Welfare reconsidered the above guideline in October 1996, and it is considered that exhaustion concentration of dioxins in refuse incineration continuous furnace which will be constructed in future should be about 0.1 ng-TEQ (Toxicity Equivalent Conversion Concentration)/Nm.sup.3 or less.
The present invention has been made in view of the problems involved with the prior art. Accordingly, an object of the present invention is to provide a method and apparatus of treating incinerator exhaust gas, which can remove hydrogen chloride gas, sulfur oxide gas, and harmful substances such as dioxins or mercury, at high efficiency by sufficiently securing contact retention time with the incinerator exhaust gas, and recover dust in a dried state, and also can rapidly cool incinerator exhaust gas having a high temperature (hereinafter referred to as "high temperature incinerator exhaust gas"), without deterioration of adsorption and absorption performances of a harmful substance removing powder.
Another object of the present invention is to decrease regeneration amount of dioxins, leading to further reduction in concentration of dioxins discharged, by limiting waste heat recovery from exhaust gas to a range capable of suppressing generation of dioxins, and also cooling high temperature exhaust gas rapidly, in particular at a temperature drop rate which passes through the vicinity of 300.degree. C. within short period of time of 1 second or less.